In recent years, in regard to a vehicle driven by an internal combustion engine, a system for automatically stopping or restarting the internal combustion engine during driving has been brought to practical use in order to meet a demand for low fuel consumption. Further, there is a need to determine whether or not the storage battery mounted on the vehicle can output electric power necessary to restart the internal combustion engine so as to positively restart the internal combustion engine after being automatically stopped.
Hitherto, in view of the need, a charging state of the storage battery is estimated when the vehicle is stopped, and when it is determined that the electric power necessary to restart the internal combustion engine can be output, the internal combustion engine is allowed to be automatically stopped (idling stop) (see, for example, Patent Literature 1).
In this case, when the internal combustion engine is restarted, a battery voltage of the storage battery serving as a power supply greatly drops due to power consumption of a starting load such as a starter. Further, the storage battery is normally used also as a power source to an ignition device and a control device for the internal combustion engine. Therefore, when the battery voltage of the storage battery greatly drops due to the restart of the internal combustion engine, the ignition device and the control device cannot be appropriately operated, with the result that the internal combustion engine cannot be normally restarted in some cases.
Therefore, in order to normally restart the internal combustion engine, it is necessary not only to secure the electric power necessary to restart the internal combustion engine, but also to maintain an amount of reduction in the battery voltage of the storage battery, which is caused by the restart of the internal combustion engine, to a level equal to or lower than a predetermined value. That is, a starting minimum voltage Vmin that is a voltage value at which the battery voltage of the storage battery becomes lowest needs to be equal to or higher than a threshold value that does not influence the restart of the internal combustion engine.
In the related art, in view of such a need, a starting current ΔImax (current value obtained by subtracting a current I0 immediately before the start, which is measured immediately before the starter is driven, from a peak current Imax measured while the starter is being driven) discharged from the storage battery is calculated and stored. Then, based on the starting minimum voltage Vmin of the storage battery, which is calculated from the stored starting current ΔImax and from the internal resistance and the battery voltage of the storage battery that are exhibited immediately before the starter is driven, determination is made as to whether or not to restart the internal combustion engine by driving the starter (see, for example, Patent Literature 2).
Further, in the related art disclosed in Patent Literature 2, in order to further increase determination accuracy, a temperature or a charging rate of the storage battery and the calculated starting current ΔImax are stored in association with each other. Then, based on the association, the starting current ΔImax corresponding to the temperature or the charging rate of the storage battery exhibited immediately before the starter is driven is estimated, and the starting minimum voltage Vmin of the storage battery is calculated.
Further, it is known that there are large fluctuations in the battery voltage of the storage battery due to depolarization when a period since a previous time at which the internal combustion engine is automatically stopped until this time at which the internal combustion engine is restarted is within a predetermined period. Therefore, in order to further increase the determination accuracy, when the above-mentioned period is within the predetermined period, the estimation of the starting current ΔImax based on the battery voltage of the storage battery is inhibited, or the starting current ΔImax is estimated by making a correction thereto in consideration of the depolarization (see, for example, Patent Literature 3).